1. a) Field of the Invention
The present invention relates to a high frequency semiconductor integrated circuit device, and more particularly to a high frequency semiconductor integrated circuit device housing a semiconductor integrated circuit chip in a package which has a plurality of package terminals.
2. b) Description of the Related Art
High frequency semiconductor integrated circuit devices such as microwave monolithic IC""s (MMIC""s) integrated with transistors such as GaAs FET""s are widely used nowadays.
MMIC is mounted, for example, on a printed circuit board.
FIGS. 6A and 6B show one example of how a conventional MMIC is mounted.
Referring to FIG. 6A, a package 51 of MMIC is mounted on a printed circuit board 55. The MMIC package 51 has eight package terminals T51 to T58. The package terminal T51 is a drain bias terminal via which a bias is applied to the drain of a transistor (not shown) directly or via a coil (not shown), and is connected to a line DB on the printed circuit board 55.
The package terminal T58 is a terminal from which a d.c. bias voltage is applied to the gate of the transistor via a resistor (not shown), and is connected to a line DC on the printed circuit board 55. A high frequency signal is applied to a package terminal T55 via an input line RF1. A high frequency signal is output from a package terminal T54 to a high frequency output line RFo.
Of the eight package terminals, high frequency signals are transferred via the package terminals T55 and T54 which are located at remotest positions relative to each other. The terminals T51 and T58 to which a d.c. voltage is applied are located at remotest positions from the terminals via which high frequency signals are transferred.
The package terminals T52, T53, T56 and T57 disposed between these terminals T51, T54, T55 and T58 are connected to a ground wiring GW.
Such a circuit layout provides isolation between high frequency wirings, for example, 50 dB isolation. The terminal T51 for supplying a bias voltage to the drain is also isolated appropriately.
FIG. 6B shows another layout providing sufficient isolation. A package terminal T55 of an MMIC package 51 is a high frequency input terminal. A nearby package terminal T56 is a floating terminal which is not connected to the integrated circuit. The next package terminal T57 may be connected or not connected to the integrated circuit.
In order to isolate the package terminal T55 via which a high frequency signal is transferred, the nearby floating package terminal T56 is electrically fixed to a ground plane GP of the package.
By electrically fixing the package terminal adjacent to the high frequency transferring package terminal, to the ground plane, isolation of the high frequency transferring package terminal is attained. For example, even if a drain bias voltage or high frequency signal is applied to the package terminal T57, interference from the package terminal T56 can be reduced.
Most of the conventional MMICs have at least one input terminal and one output terminal of a high frequency signal. However, as the integration degree of a high frequency semiconductor integrated circuit chip rises, it becomes necessary to transfer high frequency signals via more package terminals or to supply bias current or voltage to more package terminals.
As the number of active package terminals in use increases, the number of remaining inactive package terminals reduces. If these active package terminals are used without any isolation, high frequency interference between these terminals becomes serious.
For example, sufficient isolation is required between a local oscillation signal having a sharp spectrum and a modulated wave in order to maintain a precision of the modulated signal, or between input and output signals having a large level difference before and after amplification in order to prevent circuit oscillation to be caused by feedback.
As described above, as the number of package terminals actually used for circuit operation of a high frequency semiconductor integrated circuit device increases, isolation between active package terminals may become insufficient.
It is an object of the present invention to provide a high frequency semiconductor integrated circuit device capable of providing sufficient isolation between package terminals even if the number of remaining package terminals not in use reduces.
According to one aspect of the present invention, there is provided a high frequency semiconductor device comprising; a semiconductor chip constituting an integrated circuit including circuit elements of at least one transistor, one resistor and one capacitor; a package for housing the semiconductor chip in an inner space defined in the package; a ground conductor formed on the package and on the semiconductor chip; a first package terminal formed on the package, a high frequency signal being transferred via the first package terminal; a second package terminal formed on the package, the second package terminal being either a package terminal via which a high frequency signal is transferred or a package terminal via which current is supplied to a node via which a high frequency signal is transferred or to a drain of a transistor; a third package terminal formed on the package and disposed between the first and second package terminals, the third package terminal applying a bias voltage to a circuit element of the integrated circuit via a first resistor; and a first capacitor disposed in the package, one electrode of the first capacitor being connected to an intermediate point between the third package terminal and the first resistor and the other electrode being connected to the ground conductor.
The third package terminal disposed between the first and second package terminals, which inevitably receive high frequency interference if they are disposed near each other, can suppress interference. If the third package terminal is not a ground terminal but a terminal which supplies a bias voltage to a circuit element via a resistor, the first and third package terminals interfere with each other in terms of high frequency, and moreover, the third and second package terminals also interfere with each other in terms of high frequency. Therefore, interference occurs between the first and second package terminals.
Since a capacitor grounded in the package is connected to the third package terminal, high frequency components leaked from the first package terminal to the third package terminal can be absorbed to the ground via the capacitor. Accordingly, high frequency interference from the first package terminal to the third package terminal can be reduced.
Similarly, high frequency interference from the second package terminal to the first package terminal can be reduced. The third package terminal is used for applying a d.c. bias voltage to a circuit element so that even if it is grounded via the capacitor in terms of high frequency via the capacitor, the function thereof is not affected.
As described above, it is possible to use as an isolation terminal the terminal for applying a voltage to the integrated circuit via a resistor. Accordingly, sufficient isolation can be ensured even for a high frequency semiconductor integrated circuit without no empty or inactive terminal.